According to the rapid spread of electronic devices such as personal computers and cellular phones in recent years, batteries capable of charging and discharging by a large capacity are in use, e.g., nickel-metal hydride secondary batteries, lithium ion secondary batteries and lithium polymer secondary batteries. Lately, these batteries are also used as a power source of electric vehicles.
A secondary battery of this kind fundamentally comprises {circle around (1)} a metallic current collector in the form of a foil, {circle around (2)} an electrode formed by coating a material which causes electrochemical reaction in a reversible manner, i.e., a so-called active material, on the current collector, {circle around (3)} a separator for isolation between a positive electrode and a negative electrode and {circle around (4)} an electrolyte and a battery case.
In relation to such a secondary battery, the metallic current collector in the foil shape of the above item {circle around (1)} needs to have a capability of supporting the active material. As a method of improving the capability of supporting the active material, is known a method of opening a plurality of apertures in the metal foil to improve the supporting capability of the foil with a binding force between the active materials adhered to both surfaces of the metal foil (see Japanese Laid-Open Patent Publication No. HEI 11-323593).
Since the secondary batteries are used in various electronic devices as described above, they vary greatly in size and shape. In particular, for portable electronic devices which are continuously being downsized, a small button-shaped secondary battery is used. With the use of the metal foil provided with a plurality of apertures, the current collector is reduced in volume and the current collecting ability is decreased.
The negative electrode coated with the active material of the above item {circle around (2)} is manufactured by applying the active material on both surfaces of the metal foil, followed by heating for drying and sintering, and press-bonding. Therefore, the plural apertures provided to penetrate the metal foil may cause reduction in strength and the foil may be broken during the above-described steps. In order to avoid the break, it is necessary to reconstruct the manufacturing facilities in accordance with the reduction of the foil strength.